The prior application discloses a blood filter and method of filtration for removal of leucocytes from whole blood or blood fractions. The filter material is a shape-sustaining laid textile web having a thickness of at least about 1 millimeter and a bulk density of between about 0.05 and 0.4 g/cm.sup.3. The web has a plurality of interlocked, textile fibers with average deniers between about 0.05 and 0.75 and average lengths between about 3 millimeters and 15 millimeters. The textile fibers are substantially uniformly distributed in the web so as to form a matrix of the fibers with spaces between interstices of the interlocked fibers. Within those spaces are disposed a plurality of fibrillated particles of polymeric material having a surface area of between about 5 and 60 square meters per gram. The fibrillated particles have a plurality of fine fibrils which are interlocked with adjacent textile fibers such that the fibrillated particles are not substantially displaceable from the web during filtration of blood. The weight ratio of fibrillated particles to textile fibers is between about 1:99 and 40:60.
As can be appreciated, the textile fibers and the fibrillated particles must be so interlocked that significant amounts of fibers or fibrillated particles are not displaced from the filter material during filtering of blood, since displaced fibers or particles remain in the filtered blood. A significant amount of displaced fibers or fibrillated particles in the filtered blood could cause difficulties when the filtered blood is transfused into a human patient.
As disclosed in that application, for efficient and effective depletion of leucocytes from blood passing through the filter material, both the fiber geometry and the surface area of the fibers are important, and that, very importantly, the surface area must be significantly greater than the usual prior art blood filters, since otherwise the degree of leucocyte removal is not sufficient. Further, since fiber geometry and surface area are important for leucocyte depletion, the depth (thickness) of the filter material is also important. Somewhat similarly, since the bulk density of the filter material and the denier of fibers affects fiber geometry and surface area, these are also important.
However, to achieve the high surface area of the filter material required for effective leucocyte removal from blood, a critical component is that of the very high surface area fibrillated particles in the filter material. Ordinary textile fibers cannot provide such high surface areas to the filter material which surface area is required for high leucocyte removal. The fibrillated particles are somewhat elongated particles with an elongated central portion from which radiate a large number of fibrils. Generally speaking, a typical particle has an overall length of less than 1000 microns, e.g. 5-300 microns, and a width and depth of from about 0.1 to 50 microns, e.g. 0.1 to 5 microns.
As can be appreciated, it is important to ensure that the very small fibrillated particles are not significantly displaced from the filter material during filtration of blood, and, as disclosed in that prior application, this is achieved by interlocking the fibrils of the fibrillated particles with adjacent matrix textile fibers. In addition, the depth (thickness) of the filter material, the bulk density thereof and the length of the matrix textile fibers (which affects the configuration of the laid matrix textile fibers) contribute to retaining the fibrillated particles. Further, especially at higher weight ratios of fibrillated particles to matrix textile fibers, e.g. 6:94 to 10:90, permanent securing of the fibrillated particles in the filter material may be improved by use of means for adhering the matrix textile fibers and fibrillated particles to each other, such as by tackifying adhesives and especially the use of sheath/core fibers for at least part of the matrix textile fibers, e.g. a sheath of low melt temperature polymer and a core of higher temperature polymer. When the matrix textile fibers are at least in part such sheath/core fibers, during usual processing of the filter material web, the sheath softens and causes bonding, upon cooling, between the matrix textile fibers themselves and fibrillated particles, especially the fibrils thereof.